(1) Field of the Invention
This invention relates to a novel process for the preparation of indole and derivatives thereof by reacting an aniline with a 1,2-glycol.
(2) Description of the Prior Art
In the prior art, indole derivatives have long been prepared by the well-known Fischer indole synthesis in which phenylhydrazine is reacted with a compound having an aldehyde group. If the aldehyde compound is other than acetaldehyde, the aforesaid Fischer indole synthesis can be applied to obtain indole derivatives in good yield. However, if the aldehyde compound is acetaldehyde, no reaction that yields indole has been believed to take place. In order to overcome this disadvantage, there has recently been proposed an improved process which comprises reacting phenylhydrazine with acetaldehyde at an elevated temperature of from 300.degree. to 400.degree. C. in the presence of an alumina catalyst (Japanese Patent Laid-Open No. 76864/'73).
This process surely permits the reaction to proceed and brings about the formation of indole, but fails to give a satisfactory yield. Moreover, it is greatly disadvantageous in that the catalyst has so short a life as to become totally inactive after 0.5-1 hour's use.
Indole can also be prepared by another process which comprises reacting o-toluidine with formic acid to form o-methyl-N-formylaniline and then fusing it together with potassium hydroxide. However, it is usually impossible to selectively prepare o-toluidine that is used as the starting material in this process. That is, the p-isomer is always formed in an amount equal to or greater than that of the o-isomer. Thus, treatment of the isomer formed as a by-product poses a serious problem in the case of industrial production. Moreover, the handling of solids as in alkali fusion is troublesome. For these reasons, the aforesaid process cannot be regarded as suitable for industrial purposes.
Furthermore, a number of attempts have been made to synthesize indole from N-.beta.-hydroxyethylamine, but none of them are satisfactory from an industrial point of view. For example, a process which comprises effecting the reaction at 300.degree. C. in the presence of an aluminosilicate catalyst [Zhur. Obschue. Khim., Vol. 24, pp. 671-678 (1954)] gives only a very low yield of indole. A process which comprises heating the reactant together with a molten mixed salt consisting mainly of zinc chloride (Japanese Patent Laid-Open No. 57968/'73) can give a fairly high yield of indole. However, this process has the disadvantage of requiring a complicated procedure, which makes it unsuitable for industrial purposes.
As described above, a number of processes for the synthesis of indole and derivatives thereof have been proposed. However, many of them are disadvantageous in that large amounts of by-products are formed, expensive compounds are used as the starting materials, and/or lengthy and complicated procedures are required to obtain the desired products.